The tumor suppressor protein, pRB, controls cell cycle re-entry by regulating the activity of a family of transcription factors called E2F. The focus of this proposal is to investigate the processes which integrate feedback regulation among the E2F family members with the action of their upstream regulators, pRB and two pRB-related proteins, called p107 and p130. Ultimately understanding these issues should lead to an understanding of how upstream regulatory signals from this pathway are converted into a precisely regulated pattern of downstream transcriptional activation controlling cell proliferation. To date, seven genes have been identified that encode components of E2F. The products of these genes can be subdivided into two distinct classes, called E2F (1 through 5) and DP (1 and 2), whose products heterodimerize to give rise to functional E2F activity. The complex properties of the endogenous E2F activity arises from the varied properties and hence biological activities of the multiple E2F/DP heterodimers. To accurately assess these biochemical and functional activities of the E2Fs, specific monoclonal antibodies have been developed for each of the components of the E2F system. Using these specific antibodies, intriguing and unexpected observations regarding the cell cycle regulation of the endogenous E2F/DP complexes have been made. In particular, E2F-4 undergoes changes in its subcellular localization in a cell cycle dependent manner which appears to play a pivotal role in mediating the different biological properties of pRB, p107 and p130. These studies suggest that E2F-4 may play a key role in regulating the activity of the entire E2F family and hence acts as master regulator of the endogenous E2F activity and of pRB, p107 and p130 action. For these reasons, this proposal is focused on understanding how the localization of E2F-4 contributes to the regulation of the endogenous E2F activity. The specific aims are as follows: characterization of the cell cycle dependent localization of the endogenous E2F complexes, elucidation of the mechanisms by which the differential localization of the E2F complexes is mediated, and analysis of the biological functions of the E2F complexes.